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BGoel
Beginner
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Sudden change in Haswell power consumption

I am monitoring the power consumption of haswell processor and I have noticed some sudden changes in the power consumption for no apparent reason. For demonstration, as shown in the figure below (left y-axis is in Watt and right y-axis is in Celsius), I have fixed the cpu C-state for all cores to C0 at a fixed frequency and then I increase the temperature using a hair dryer. I measure the power consumption at CPU ATX power rail and using RAPL. As one can see, the power rises linearly in correlation with the temperature till about 16.5W and then there is a sudden jump to ~18.5W. This jump is visible in both ATX and RAPL values. Similarly, when the processor is allowed to cool down, the power value decreases steadily till ~17W and then drops suddenly to ~15W. There is no change in CPU/cache activity during this and the results are consistent across different runs. A similar jump is seen at the mark of around 23-24W and 35-36W. The only explanation that I can come up with is that this is because of phase change in Haswell's internal voltage regulator. Can someone confirm if this a fair assumption?

 

haswell_power_jump.jpg

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